Widely known is a method of gas cleaning (A. G. Amelin "Sulphuric Acid Production Process" 1967, Khimiya Publishers, Moscow, pp. 135-136) pursuant whereto a gas is scrubbed with a 20-30% sulphuric acid at 60.degree.-70.degree. C. in a countercurrent flow in a single-stage packed tower formed wherein is a gas/liquid mixture. The acid channeled through the tower packing evaporates, and the gas cools down. Further cooling takes place in a shell-and-tube gas cooler. A condensate which forms therein drips from the tubes into a tank for collecting liquid mixture. The gas outflow passes through a first electric precipitator, another gas cooler and a second electric precipitator which are all successively interconnected. The droplets of mist comprising the gas/liquid mixture entering each of the electric precipitators absorb the ions of gas formed in the electric field, receive and electric charge and move towards collecting electrodes where they lose the charge and drip as a condensate film down the collecting electrodes into a tank for liquid mixture.
Alsoi widely known is an apparatus materializing the method of gas cleaning (A. G. Amelin "Sulphuric Acid Production Process", 1967, Khimiya Publishers, Moscow, pp. 135-136) comprising a scrubbing tower, pumps for feeding scrubbing fluid, a first gas cooler, a first electric precipitator, a second gas cooler and a second electric precipitator which are all connected in series by pipes.
The scrubbing tower is provided with nozzles at the top admitted wherethrough is a scrubbing fluid. A settling tank for separating solid matter from liquid and a tank for collecting the scrubbing fluid are provided at the bottom of the tower. The gas coolers are of the shell-and-tube type, the cooling water passing through the tubes. Each of the electric precipitators consists of a housing with gas inlet and outlet connections in a side wall and condensate and scrubbing fluid draining connections at the bottom. A header with nozzles fed wherethrough is an electrode-flushing fluid is provided at the top. Vertical collecting electrodes of the tubular type are provided inside the housing, being fitted to a busher connected to a source of power. Coaxial discharge electrodes in the form of wires held taut by weights at their lower ends are provided in the bores of the collecting electrodes. The discharge electrodes are connected to a frame provided at the top of the housing.
The countercurrent flow of the gas and scrubbing fluid through the scrubbing tower fails to make for an adequate heat and mass transfer between the gas and fluid, impairing cleanliness of the gas. Solids settle on the cooling surface of the gas cooler, reducing the rate of heat transfer and impairing cleanliness. The velocity of the gas flow through the scrubbing tower and electric precipitators is low so that large-volume equipment is required for good results. Nonuniform wetting of the suspended matter not only has an adverse effect on the scrubbing process but increases the friction losses in the scrubbing tower and the power requirements for inducing the gas flow.
Also known is a method of gas cleaning (B. T. Vasiliev, M. I. Otvagina "Sulphuric Acid Technological Process" 1985, Khimiya Publishers, Moscow, pp. 288-289) according whereto a gas containing 7-8% sulphur dioxide is scrubbed with sulphuric acid in two stages. The first-stage scrubber is a Venturi tube with a cocurrent flow pattern. The gas admitted into the Venturi tube at a velocity of 40-50 m/s disperses the scrubbing fluid into droplets. This surface of contact between the gas and scrubbing fluid increases, and the gas cools down to the temperature of its adiabatic saturation. A sulphuric acid mist is formed; fumes of arsenic and selunium condense and coagulate, being then precipitated by the droplets of the scrubbing fluid and dissolved in the sulphuric acid mist. The droplets of scrubbing fluid also precipitate dust.
The second-stage scrubbing is effected in a packed tower employing the countercurrent flow pattern. The gas/liquid mixture admitted into the tower at a velocity of 0.4-0.5 m/s contacts the scrubbing fluid at the wetted surface of the ceramic rings used as the packing material. The gas cools down to 30.degree.-40.degree. C. Further precipitation of the arsenic and selenium fumes takes place in the tower due to contact with the droplets of scrubbing fluid. Electric precipitation of the unwanted matter is effected at a field strength of 1-3 kV/cm, the velocity of the gas/liquid mixture flow being 0.8-1 m/s in the presipitator.
The ions of the gas which is ionized on the space between the collecting and discharge electrodes of the electric precipitator combine with the particles of condensed fumes of the unwanted matter and the charged particles settle at the collecting electrodes in the form of a liquid film which is disposed of from the precipitator.
Further known is also an apparatus which materializes the method of gas cleaning (V. T. Vasiliev, M. I. Otvagina "Sulphuric Acid Technological Process", 1985, Khimiya Publishers, Moscow, pp. 288-289) and comprises a Venturi tube, a packed scrubbing tower and two electric precipitators which are interconnected by a common fluid flow. The Venturi tube has consecutively located a convergent cone with nozzles admitted wherethrough is the scrubbing fluid, a throat and a divergent cone. The nozzles of the scrubbing tower are connected to pumps for feeding the scrubbing fluid and to a cooler. The Venturi tube and tower are connected by a fluid flow to a settling tank separating solids from the liquid. Each electric precipitator consists of a housing with gas inlet and outlet connection located at the top and bottom, respectively. A frame connected to a source of power is installed at the top of the housing. Perforated plates are fitted to a side wall of the housing at the top and bottom, and located between the plates and connected thereto are vertical tubular collecting electrodes. They are spaced from one another by a distance equalling 1.4 to 1.8 times their diameter. Coaxial discharge electrodes made of wire with a starry cross section are located in the bores of the collecting electrodes, being attached to the frame at the top and held taut by weights at the lower ends.
The countercurrent flow of the gas and scrubbing fluid in the packed tower fails to wet the packing material uniformly. This has an adverse effect on the rate of heat and mass transfer between the gas and scrubbing fluid and impairs cleanliness. The velocity of gas flow is low in the tower and its volume must be large. The metal requirements for manufacturing the towers and consequently high. The scrubbing of a gas with a variable content and volume is difficult, for, e.g. if the velocity and volume of the gas flow through the packed tower increases, the gas will contact the scrubbing liquid inside the void tower space rather than at the surface of the packing material. A heavy carry-over of the droplets of scrubbing liquid leading to an impaired cleanliness is unavoidable in this case. The electric precipitators operate at a low field strength (1-3 kV/cm), at a low velocity of the gas flow (0.6-0.8 m/s) and at a high gas temperature (30.degree.-35.degree. C.). The process variables cannot be increased in value due to wide gaps between the collecting and discharge electrodes and extending spacing of the longitudinal axes of the collecting electrodes. Widely spaced collecting electrodes reduce the surface precipitated whereat is the solid matter. This impairs cleanliness, especially that of dust-laden gases with a dust content of up to 0.2 g/m.sup.3. The gas flow must be interrupted from time to time in order to flush the electrodes. This adds to the capital outlays and operational costs.